In hybrid vehicle architectures such as Plug-in Hybrid Vehicles (PHEV), Range Extended Electric Vehicles (REEV), or Series Hybrid Vehicles (SHV), multiple independent sources of energy are available to propel the vehicle. Energy sources include internal combustion (“IC”) engines and motor/generators powered by energy storage systems such as high voltage batteries, super capacitors, etc. In all of these architectures, vehicle propulsion can be achieved by using just the stored electric energy to run the motor, or the engine can be used to propel the vehicle (assuming the engine is mechanically coupled to the driveline) and/or to work with a generator to provide propulsion power to the driveline and to replenish the energy storage system. Depending on the drive cycle and battery size, the engine can be shut off and remain stopped for significant portions of the drive cycle. However, this could also pose a challenge, since starting a cold engine requires more energy and subsequently, engine warm-up inefficiencies can consume a significant amount of fuel, thereby reducing the overall fuel economy of the vehicle.
If the driving route does not provide adequate opportunities to replenish the energy storage device through regenerative braking or the occasional charging station, the engine may be restarted to replenish the energy storage device more frequently. However, frequent starting and stopping of the engine can cause undesirable vehicle noise, vibration and harshness, reduced engine hardware life expectancy, etc. Other considerations include municipal ordinances which may limit or constrain engine running. Also, for vehicles used in repetitive routes (e.g., bus routes, etc.), daily variability in route conditions may result in some days where the stored battery charge is sufficient to complete the whole day, and starting the engine prematurely is wasteful. In some applications, the battery may be sized such that it does not provide sufficient power to climb the steepest route grades and the engine must be brought online to provide supplemental electric power. The varying number of times the engine may be started and the varying duration of total engine run time on a given day, combined with the efficiency consequences of engine warm-up in cold environments can produce sub-optimal fuel economy if not managed properly. Hence, an approach is needed to address these issues, while not affecting the ability of the powertrain to meet driver demand.